Thermal control units (TCUs), such as heating and chilling systems are widely used to establish and maintain a process tool or other device at a selected and variable temperature. Typical examples of a modem thermal or temperature control unit are found in highly capital intensive semiconductor fabrication facilities. Stringent spatial requirements are placed on the TCUs, in order to preserve expensive floor space as much as possible. Reliability must be assured, because the large capital equipment costs required do not tolerate downtime in operation if profitable performance is to be obtained. The target temperature may be changed for different fabrication steps, but must be held closely until that particular step is completed. In many industrial and common household refrigeration systems, the purpose is to lower the temperature to a selected level, and then maintain the temperature within a temperature range that is not highly precise. Thus even though reliable and long-lived operation is achieved in these commercial systems, the performance is not up to the demands of highly technical production machinery.
In most modern TCUs, actual temperature control of the tool or process is exercised by use of an intermediate thermal transfer fluid which is circulated from the TCU through the equipment and back again in a closed cycle. A thermal transfer fluid is selected that is stable in a desired operating range below its boiling temperatures at the minimum operating pressure of said fluid, and must also have suitable viscosity and flow characteristics within its operating range. The TCU itself employs a refrigerant, usually an ecologically acceptable type, to provide any cooling needed to maintain the selected temperature. The TCU may circulate the refrigerant through a conventional liquid/vapor phase cycle. In such cycles, the refrigerant is first compressed to a hot gas at high pressure level, and then condensed to a pressurized liquid. The gas is transformed to a liquid in a condenser by being passed in close thermal contact with a cooling fluid; where it is either liquid cooled by the surrounding fluid or directly by environmental air. The liquid refrigerant is then lowered in temperature by expansion through a valve to a selected pressure level. This expansion cools the refrigerant by evaporating some of the liquid, thereby forcing the liquid to equilibrate at the lower saturation pressure. After this expansive chilling, the refrigerant is passed into heat exchange relation with the thermal transfer fluid to cool said thermal transfer fluid, in order to maintain the subject equipment at the target temperature level. Then the refrigerant is returned in vapor phase to the pressurization stage. A source of heating must usually be supplied to the thermal transfer fluid if it is needed to raise the temperature of the circulated thermal transfer fluid as needed. This is most often an electrical heater placed in heat exchange with the circulated fluid and provided with power as required.
Such TCUs have been and are being very widely used with many variants, and developments in the art have lowered costs and improved reliability for mass applications. In mass produced refrigerators, for example, tens of thousands of hours of operation are expected, and at relatively little cost for maintenance. However, such refrigeration systems are seldom capable of operating across a wide temperature range, and lower cost versions often use air flow as a direct heat exchange medium for the refrigerated contents.
The modern TCU for industrial applications has to operate precisely, e.g., a typical requirement being .+/−.<1. degree. C., at a selected temperature level, and shift to a different level within a wide range (e.g. −40.degree. C. to +60.degree. C. for a characteristic installation). Typical thermal transfer fluids for such applications include a mixture of ethylene glycol and water (most often in deionized form) or a proprietary perfluorinated fluid sold under the trademark “Galden” or “Fluorinert”. These fluids and others have found wide use in these highly reliable, variable temperature systems. They do not, however, have high thermal transfer efficiencies, particularly the perfluorinated fluids, and impose some design demands on the TCUs. For example, energy and space are needed for a pumping system for circulating the thermal transfer fluid through heat exchangers (HEXs) and the controlled tool or other equipment. Along with these energy loss factors, there are energy losses in heat exchange due to the temperature difference needed to transfer heat and also losses encountered in the conduits coupling the TCU to and from the controlled equipment. Because space immediately surrounding the device to be cooled is often at a premium, substantial lengths of conduit may be required, which not only introduces energy losses but also increases the time required to stabilize the temperature of the process tool. In general the larger the volume of the TCU, the farther the TCU needs to be located remotely from the device to be controlled. The fluid masses along the flow paths require time as well as energy to compensate for the losses they introduce. Any change in temperature of the device to be controlled must also affect the conduits connecting the TCU and the controlled device along with the thermal transfer fluid contained in said conduits. This is because the thermal transfer fluid is in intimate thermal contact with the conduit walls. Thus, the fluid emerging at the conduit end nearest the controlled device arrives at said device at a temperature substantially equal to that of the conduit walls, and these walls must be changed in temperature before the controlled device can undergo a like change in temperature.
To the extent that straightforward refrigeration systems may have in the past employed a refrigerant without a separate thermal transfer fluid, it has been considered that the phase changes imposed during the refrigeration cycle prohibit direct use of the refrigerant at a physical distance outside the cycle. A conventional refrigerant inherently relies on phase changes for energy storage and conversion, so that there must also be a proper state or mix of liquid and vapor phases at each point in the refrigeration cycle for stable and reliable operation of the compressor and other components. Using a saturable fluid such as a refrigerant directly in heat exchange with a variable thermal load presents formidable system problems.
Various systems for temperature control have been proposed that depart from the traditional two phase vapor cycle, including those described in U.S. Pat. No. 7,178,353 and U.S. Pat. No. 7,415,835 to inventors Kenneth W. Cowans et al. This departure is directed to a novel temperature control system which combines flows of refrigerant in a hot gas pressurized mode with the same refrigerant in an expanded vapor/liquid mode. The system combines some expanded refrigerant flow with a suitable proportion of pressurized hot gas in a closed circuit vapor-cycle refrigeration system. The combined refrigerant stream generated can exchange thermal energy directly with a load, as in a heat exchanger (HEX). Such systems offer substantial benefits in improving heat transfer efficiency and economy and in enabling rapid and precise temperature level changes. Since they require no intermediate coolant and the pressure can be varied rapidly, this approach, which for succinctness has sometimes been termed TDSF for “Transfer Direct of Saturated Fluids,” offers distinct operative and economic advantages for many temperature control applications.
U.S. Pat. No. 7,415,835 assigned to the present assignee, the contents of which are fully incorporated hereby by reference, introduced a system that employs the high thermal transfer efficiency of a refrigerant mixture of liquid and vapor in a system capable of very fast temperature change response. A benefit of that system was that it eliminated the need for substantial delay times to correct temperature levels at the device being controlled, as well as for substantial energy losses in conduits and heat exchangers, and the need for substantial time delays in shifting between target temperatures at different levels.
The trapped ramp system employs four modes in its operation: Ramp-up, Regulation, Stand-by, and Ramp-down. In the Ramp-up mode, the electrostatic chuck is heated rapidly from one regulated temperature to a higher temperature. In the Regulation phase, a large amount of radio frequency (RF) energy is cooled during processing. The electrostatic chuck is regulated in the Stand-by phase at a temperature but the system is called on to supply heat. In the Ramp-down mode, the electrostatic chuck is cooled rapidly from one regulated temperature to a lower temperature.
U.S. patent application Ser. No. 13/651,631 to Cowans et al., incorporated fully herein by reference, discusses improvements in vapor cycle systems used for refrigeration or heat exchange that can be realized by modifying the conventional vapor cycle (FIG. 2), having to incorporate an additional thermal exchange step after expansion of compressed condensed refrigerant (FIG. 3). This interchange of thermal energy is then between the expanded refrigerant and the return flow from the evaporator and is accompanied by a controlled pressure drop, which introduces enhanced post condensing (EPC). The post condensation lowers the quality level (ratio of vapor mass to total mass) of refrigerant delivered to the evaporator and raises the effective heat transfer coefficient during energy exchange with the load. This expedient increases the bulk density of the mass moving through the evaporator and lowers the pressure drop introduced, minimizing heat transfer losses in the low efficiency region of the evaporator. The controlled pressure drop, provided by a pressure dropping device, introduces a substantially constant pressure difference to assure that no expanded vapor and liquid flows during those times when maximum heating is desired.
The expanded liquid/vapor mix feeds pressurized input to one side of a two-phase heat exchanger prior to the evaporator; the heat exchanger also receives a flow of output derived from the evaporator after having serviced the load. A pressure dropping valve introduces a temperature drop of the same order of magnitude in the two-phase mixture as the mass superheat used to regulate the cooling temperature with the thermal expansion valve. This temperature drop thusly created drives heat to pass from one flow in the heat exchanger to the other flow. Consequently, by introduction of a relatively small heat exchanger and a pressure dropping device in a given temperature control unit an overall gain in H is achieved. This results in a net gain in efficiency.
Application of this principle to TDSF systems employs the flow of fluids through a supplemental heat exchanger that is generally relatively smaller than the load, and also employs a pressure dropping valve to make a temperature difference available to drive heat across said supplemental heat exchanger so as to introduce further condensation. This combination uniquely effects TDSF system operation by acting to limit and smooth out deviations in temperature changes as well as increasing system efficiency. Small changes in temperature level can be introduced by precise valve regulation of the flow of hot gas into the mixture.
If a slightly higher temperature is needed and/or operation is to be at a low flow or power level, the situation is different, because the pressurized hot gas source presents a much larger potential energy input (than does condensed liquid vapor input after expansion) so that stability and precision can be problematic if temperature is to be raised a relatively small amount. In this situation, employment of enhanced post condensation is effective in changing the flow rate of pure gaseous medium at high pressure so that the control of temperature becomes much more precise particularly at higher temperatures where it may be necessary to heat and cool alternately in order to control temperature. The heat exchanger and pressure dropping valve in the flow path compensate for nonlinearity in thermal energy exchange by smoothing the rate of change of temperature increase and ensuring thermodynamic balance. Employing EPC in the TDSF context, therefore, assures that a higher, stable temperature level can be attained more rapidly regardless of the increment of change and the power level involved.
FIG. 3 illustrates the repumping mechanism consistent of a check valve and pump plumbed in between the input and output of an evaporator in a vapor-cycle system. The pump is used when it is desirable or necessary to increase the heat transfer coefficient within the evaporator. When the pump is not turned on, the vapor cycle system functions as if the repumping system was not installed. In FIG. 4, both the repumping system is used with the enhanced post-condensing. In the combined system, the repumping is turned on when the output at the evaporator is changing rapidly from one temperature to another. In this ramping process, the enhanced post condensing enhancement of efficiency, particularly on a vapor cycle system that has been retrofitted with an EPC system that includes a smaller compressor, may not increase the speed of ramping. This is because the smaller compressor will flow less mass across the evaporator and thus have a smaller heat transfer coefficient, particularly while the load temperature is being changed.
FIG. 5 shows a graph documenting data about the heat transfer coefficient within the evaporator of a vapor cycle refrigerator or heat pump using the refrigerant R22, which is representative of other refrigerants. The data shows how the enhanced post condensing augments the vapor cycle efficiency. The function of the EPC is to eliminate the sharp drop off of the heat transfer coefficient with a two-phase quality of around eighty percent (80%) or more. As seen in FIG. 5, the heat transfer coefficient is very sensitive to the mass velocity within the evaporator. The characteristic of the curves shown in FIG. 5 illustrate the effect of velocity. As the liquid boils to gas the velocity increases due to the fact that the gas phase is considerably less dense. As a result, FIG. 5 shows a monotonically increasing heat transfer coefficient as quality increases due to liquid being boiled to a gas until quality exceeds about 80%. Thereafter, the heat transfer quality drops precipitously, becoming equal to that of pure gas at the outlet of the conventional evaporator.
The vapor cycle is used as the driving system in a temperature control unit such as that discussed above. The temperature control systems based on the principles discussed in U.S. Pat. No. 7,178,353 and U.S. Pat. No. 7,415,835 discussed above, refer to the transfer direct of saturated fluids, or TDSF. The TDSF is the basis, in turn, for the trapped ramp (TR) system set forth in U.S. patent application Ser. No. 13/651,631 (discussed above). That is, the trapped ramp system is based, for heating an electrostatic chuck rapidly up to a high temperature, on a TDSF using a stream of hot high pressure gas condensing within an electrostatic chuck, flowing from said electrostatic chuck through a valve that opens On Rise of Input Temperature (ORIT valve, or “ORIT”) which thereafter regulates the temperature of the electrostatic chuck. It regulates the temperature by controlling pressure due to the inherent nature of saturated fluids.
As the trapped ramp system is used to rapidly heat (ramp up) the load, the condensing gas will not flow through until the pressure ahead of the ORIT reaches regulated temperature. This can cause the fluid to back up within the load, thereby diminishing the area available for condensing the gas. As a consequence, the rate of heating slows. The repumping system counteracts this deceleration. As the pump of the repumping activates, it forces a flow through the load, in this case the electrostatic chuck. In turn, this action allows the incoming hot gas to condense as it passes through the electrostatic chuck, thus allowing for more rapid heating.